From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oa0-f43.google.com (mail-oa0-f43.google.com [209.85.219.43]) by kanga.kvack.org (Postfix) with ESMTP id 0E5446B0036 for ; Tue, 17 Jun 2014 18:38:22 -0400 (EDT) Received: by mail-oa0-f43.google.com with SMTP id o6so52219oag.2 for ; Tue, 17 Jun 2014 15:38:21 -0700 (PDT) Received: from g4t3426.houston.hp.com (g4t3426.houston.hp.com. [15.201.208.54]) by mx.google.com with ESMTPS id bg7si35519obb.13.2014.06.17.15.38.21 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Tue, 17 Jun 2014 15:38:21 -0700 (PDT) From: Waiman Long Subject: [PATCH v2 1/2] mm, thp: move invariant bug check out of loop in __split_huge_page_map Date: Tue, 17 Jun 2014 18:37:58 -0400 Message-Id: <1403044679-9993-2-git-send-email-Waiman.Long@hp.com> In-Reply-To: <1403044679-9993-1-git-send-email-Waiman.Long@hp.com> References: <1403044679-9993-1-git-send-email-Waiman.Long@hp.com> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton , Mel Gorman , Rik van Riel , Ingo Molnar , Peter Zijlstra , "Kirill A. Shutemov" Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Scott J Norton , Waiman Long In the __split_huge_page_map() function, the check for page_mapcount(page) is invariant within the for loop. Because of the fact that the macro is implemented using atomic_read(), the redundant check cannot be optimized away by the compiler leading to unnecessary read to the page structure. This patch moves the invariant bug check out of the loop so that it will be done only once. On a 3.16-rc1 based kernel, the execution time of a microbenchmark that broke up 1000 transparent huge pages using munmap() had an execution time of 38,245us and 38,548us with and without the patch respectively. The performance gain is about 1%. Signed-off-by: Waiman Long --- mm/huge_memory.c | 4 ++-- 1 files changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index e60837d..be84c71 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1744,6 +1744,8 @@ static int __split_huge_page_map(struct page *page, if (pmd) { pgtable = pgtable_trans_huge_withdraw(mm, pmd); pmd_populate(mm, &_pmd, pgtable); + if (pmd_write(*pmd)) + BUG_ON(page_mapcount(page) != 1); haddr = address; for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) { @@ -1753,8 +1755,6 @@ static int __split_huge_page_map(struct page *page, entry = maybe_mkwrite(pte_mkdirty(entry), vma); if (!pmd_write(*pmd)) entry = pte_wrprotect(entry); - else - BUG_ON(page_mapcount(page) != 1); if (!pmd_young(*pmd)) entry = pte_mkold(entry); if (pmd_numa(*pmd)) -- 1.7.1 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org